The broad, long-term objective of this project is to understand the complex molecular recognition events that govern the selective interactions between superantigens (SAGs) and T cell receptors (TCRs) that are responsible for SAG-mediated disease and death. This will provide a critical knowledge platform from which to effectively engineer TCR protein fragments with markedly improved affinities for SAGs that are capable of inhibiting SAG-mediated disease. These studies will, additionally, provide model systems from which we will gain significant insight into fundamental aspects of protein-protein interactions and the initiation of T cell activation by SAGs. We will address the following Specific Aims: (1) Elucidate the molecular basis of wild type SAG-TCR selectivity, cross-reactivity and fine specificity;(2) Develop TCR V[unreadable] domain-derived SAG antagonists through integrative molecular design;and (3) Define the molecular basis of enhanced affinity in SAG-TCR V[unreadable] domain variant complexes. SAGs comprise a class of toxins that elicit massive T cell proliferation through simultaneous interactions with class II major histocompatibility complex and TCR molecules. This results in the stimulation of up to 20 percent of the entire T cell population, leading to the release of massive amounts of pyrogenic and inflammatory cytokines, and subsequently, toxic shock syndrome, among other human diseases. Superantigens (SAGs), on account of their broad T cell activation and resulting massive cytokine production, have been implicated the pathogenesis of numerous human diseases, including toxic shock syndrome, food poisoning and autoimmune disorders. Due to their extreme virulence and the relative ease with which they could reproduce and spread throughout a population, SAGs represent credible candidates for biological weapons of mass destruction, and have been classified as Category B Select Agents by the Centers for Disease Control and Prevention. The development of therapeutics to treat SAG-mediated disease, against which no drug or vaccine exists, is therefore critical to both improved public health and national defense against bioterrorism.